<h2>Problem 172</h2>
<div style="color:#666;font-size:80%;">15 December 2007</div><br />
<div class="problem_content">
<p>How many 18-digit numbers <var>n</var> (without leading zeros) are there such that no digit occurs more than three times in <var>n</var>?</p>
</div><br />
